Technology is not something you research and then have forever.
It is bound to specialists and to the ability to feed these specialists so their trade is carried on.
Smiths in the middle ages is a small community. Metallurgists or people able to make good steel are even rarer.
If those get killed, you lose the knowledge unless you have a written knowledge base in which case you only loose the practical experience.
Chances are good that the Mountain Clans either simply lost their specialists or were unable to sustain them.
Add in the fact that they are surrounded by metal-using societies from who they can trade and steal it there would be little reason to use their scarce ressources to try to prop up their own smith caste.
Same goes for the North beyond the Wall because their is hardly any food.
In order for people to spend their entire lifes in the pursuit and learning of metallurgy and the art of weapon crafting you need a food surplus. You need even more of a food surplus to feed the miners who dig out the ore out of the earth (someone mentioned bog iron but this tends to be low grade and is also only a local phenomenon, e.g. Europe had it, America did not).
The problem with gouging First Men's technology advance by looking at supposed 'remnants' is the same as trying to work out our tech by looking on a refugee column without any possesions.